Biosensors are a measurement device utilizing excellent molecular recognition ability of living organisms or biomolecules. As pairs of affinity binding partners in living organisms, for example, enzyme-substrate, antigen-antibody, and DNA-DNA are known. Biosensors utilize the principle that one of these pairs can be selectively measured by using the other of the pair immobilized or supported on a substrate. Recently, biosensors have been expected to be broadly used not only in the field of medicine but also in the fields of environment and food. Consequently, in order to broaden the application fields of biosensors, highly sensitive and highly efficient biosensors which can be installed at every place or which are reduced in size and weight so as to be portable are expected.
As one of methods detecting such biomolecular interactions, the magnetic detection method utilizing a magnetic label is now under active development and used in solid-phase analysis.
FIG. 1 illustrates an example of the conventional solid-phase analysis using a magnetic label. In the method shown in FIG. 1, a primary capturing body 3 (called a primary antibody when an antigen is detected by utilizing an antigen-antibody reaction) which can specifically recognize and capture a region (called an epitope when the target substance is an antigen and the antigen is detected by utilizing an antigen-antibody reaction) of a target substance 5 is previously immobilized on a surface of a substrate 1. Then, a sample solution containing the target substance 5 is brought into contact with the surface of the substrate 1. With this process, the target substance 5 is specifically captured by the primary capturing body 3. Then, a magnetic label 9 provided with a secondary capturing body 4 (called a secondary antibody when an antigen is detected by utilizing an antigen-antibody reaction) is added to the sample solution. The secondary capturing body 4 can specifically recognize and capture a region, of the target substance 5, other than the region which is specifically captured by the primary capturing body 3. (Here, the magnetic label 9 includes a magnetic structure 2 and a secondary capturing body 4 immobilized on the surface of the magnetic structure 2.) With this process, the secondary capturing body 4 recognizes and captures the target substance 5 specifically captured by the primary capturing body 3 immobilized on the surface of the substrate 1. Consequently, the magnetic label is apparently captured by the target substance. Accordingly, as shown in FIG. 1, the magnetic label is immobilized in the vicinity of the surface of the substrate 1 via the target substance 5.
In addition, as a method different from the above, the following method is also known. A magnetic label 2 provided with a secondary capturing body 4 is added to a sample solution containing the target substance 5 to form a complex of “the target substance and the secondary capturing body on the magnetic label”. Then, the resulting complex is brought into contact with a primary capturing body 3 immobilized on a substrate 1. As a result, as shown in FIG. 1, the magnetic label can be immobilized on the surface of the substrate via the target substance.
Lastly, the number of the magnetic label immobilized on the surface of a detecting element is measured by any method and thereby the number or concentration of the target substance to be determined can be calculated.
As a target-substance detecting element using such a magnetic detection technology, Japanese Patent Application Laid-Open No. 2001-033455 discloses an immunoassay for detecting a target substance by using a magnetic material as a label. The label is bound to the target substance in a sample solution by an antigen-antibody reaction and is magnetized and detected using a superconducting quantum interference device (SQUID) as a magnetic sensor.
Further, International Publication No. WO 03/067258 discloses a biosensor for analyzing an object to be measured using detecting elements for detecting a magnetic field produced by bound magnetic molecules and having semiconductor hall devices. The analysis is conducted based on the amount of the specified magnetic molecules.
U.S. Pat. No. 5,981,297 discloses a method for detecting a magnetic signal of fine magnetic particles using a magnetoresistive element. A primary capture molecule on a sensor element is bound to a secondary capture molecule labeled with the fine magnetic particles as a signal via a target molecule.
The above-described methods are biosensing methods utilizing magnetic labels. In the meantime, Japanese Patent Application Laid-Open No. 2005-91014 discloses a biosensing method using a substrate provided with a biomolecule-immobilizing region and a template region surrounding the biomolecule-immobilizing region. The template region is covered with a monomolecular layer which does not react with the target molecule and a capture molecule. Such a structure is aimed to stably generate a signal which is derived from biomolecular interaction between a target molecule and a capture molecule and to detect the signal with a high accuracy and high sensitivity.
However, biosensors still have challenges to achieve further higher sensitivity and accuracy (a decrease in noise) and to reduce reaction time, namely, to improve the detection efficiency.